Seawater application to induce moderate stress and improve fruit quality in field-grown satsuma mandarin trees

To apply the beneficial results obtained for potted citrus trees to field-grown ones, the effects of seawater application on soil electrical conductivity (EC), water relations, and fruit quality in field-grown satsuma mandarin (Citrus unshiu Marcow.) trees were determined. In 2010, periodical applications of smaller amounts of half-strength-diluted (1/2X) or undiluted seawater (1X) delayed the reduction of leaf water potential to the objective level at 0.3 to 0.5 MPa lower than that in the control, resulting in an insignificant increase in soluble solids content (SSC). Thus, half of the total amount of 1X per area applied in 2010 was irrigated once or twice in September in 2012 and 2013. Soil EC in 1X markedly increased after just the first application at above 1.8 dS・/m and was kept at a significantly higher level than in the control until harvest, although it gradually decreased by leaching due to rainfall. Leaf water potential at predawn was reduced by 1X and the objective value was achieved from early-October and mid-September to harvest in 2012 and 2013, respectively. SSC was higher in 1X than the control throughout the experimental period and the difference between 1X and the control at harvest was 1.4 and 1.2°Brix in 2012 and 2013, respectively. Other fruit quality parameters including titratable acidity (TA) were not significantly affected by seawater irrigation in either year, except for fruit size which was slightly inhibited in 1X. No difference was observed in the leaf chlorophyll index and abscission between 1X and the control, although the Na content in the leaves was increased in 1X. These results suggest that application of relatively higher amounts of undiluted seawater in the early stage of maturation could induce moderate salt or water stress through the inhibition of water absorption by roots and improve fruit quality by increasing SSC without any visible leaf injury in field-grown satsuma mandarin trees.